“Witnesses may forget through the years but the dead, those skeletons, they don’t forget. Their testimony is silent but it is also very eloquent.”

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Identified: Mengele skull

Snow travelled the world giving a voice to the voiceless – such as the Kurds gassed by Saddam Hussein and Chemical Ali, victims of Rwanda’s genocide, the civilians dumped in mass graves by Argentina’s military death squads, the men and boy victims of US serial killer John Wayne Gacy and those of 9/11.

In a five-decade career, Snow helped reconstruct the face of Egyptian pharaoh Tutankhamun, identify air crash victims and change safety procedures based on how they died and, following the assassination of JFK, he testified before Congress that hospital X-rays were indeed those of the President.

But as fellow scientists paid tribute to “the Sherlock Holmes of bones” this week they told how his pioneering skills were originally honed on Mengele’s skull.

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Evil: Josef Mengele

Mercedes Doretti, who worked with Snow as a student and now runs the Argentine Forensic Anthropology Team, said: “Thirty years ago most human rights investigations were based on testimonial evidence.

"He started applying forensic evidence. I think he was very moved by all these families who were asking for his help.”

In June 1985 Snow was contacted by the Simon Wiesenthal Center and asked to travel to Brazil.

In 1960 Nazi hunter Wiesenthal had helped Israeli spies track down Holocaust mastermind Adolf Eichmann, who was arrested in Buenos Aires. Mengele had been in the city at the same time but managed to evade Israel’s Mossad secret service agents and flee.

A quarter of a century later in a cemetery near Sao Paulo, police uncovered the remains of a man named Wolfgang Gerhard – rumoured to be Mengele.

By comparing measurements of the bones to vital statistics and uniform sizes in Mengele’s SS records Snow was sure it was the Angel of Death, who suffered a massive stroke while out swimming and drowned.

He explained: “There was a mountain of evidence – it was overwhelming.”

But the breakthrough came when German colleague Richard Helmer helped Snow develop a technique he called “skull-face superimposition”.

They used clay to secure 30 pins to various “anatomical landmarks” around the skull, like eye position, earholes, the shape of the nose and nostrils.

They then identified the same points on photos of Mengele and superimposed the images over the skull. They were identical – “a perfect wrapping of life over death”.

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Fugitive: Purported shot of Mengele, left, in Brazil

“Everything pointed to Mengele,” Snow said later. “But now we were able to prove beyond scientific doubt that the remains were indeed his.”

Eichmann was flown to Jerusalem, tried, then executed in May 1962. He was condemned by the accounts of living witnesses but the work on Mengele’s remains would mean that murderers could be convicted by the testimony of victims from their graves.

Eyal Weizman, co-author of the book Mengele’s Skull: The Advent of Forensic Aesthetics, explained: “It proved a legal and technological turning point.

“It was the Mengele investigation that helped consolidate the process for the identification of missing people. Scientists began to appear in human rights cases as expert witnesses.”

As Snow put it: “It’s not the role of science to put the bad guys in jail but to even-handedly collect the evidence.”

In Argentina he trained students to help him excavate a mass grave. They found 500 skeletons – some of 30,000 civilians who disappeared in the seven-year “dirty war”.

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Witness: Snow at Saddam Hussein trial in 2006

As chief witness at the trial of generals and admirals he helped convict five dictators. In 2006 Snow testified against Saddam Hussein and Ali Hassan al-Majid, or “Chemical Ali”, at their trial for genocide.

The two men personally cross-examined Snow but he was unfazed. Ali had some good questions, Snow recalled later, but Saddam “was kind of scattered”.

With his Texan drawl and cowboy boots, striking Snow chain-smoked Camel cigarettes and cultivated a morbid sense of humour.

“There are 206 bones and 32 teeth in the human body,” he liked to say, “and each has a story to tell.”